A wavelength division multiplexing (WDM) optical transmission system optically transmits a plurality of wavelengths of signal light by using a single optical transmission line, thereby being able to transmit a large volume of information. The WDM optical transmission line system has been required to have a larger volume, whereby the use of not only C band (1.55-μm wavelength band) and L band (1.59-μm wavelength band) but also S band (1.50-μm wavelength band) has been under consideration.
Meanwhile, an Er-doped optical fiber amplifier (EDFA: Erbium-Doped Fiber Amplifier) using an optical fiber having an optical waveguide region doped with Er element can optically amplify signal light in C band or L band with a high efficiency. Therefore, the signal light in C band or L band can optically be amplified by an EDFA provided within a repeater, so as to be transmitted over a long distance. On the other hand, the EDFA cannot optically amplify signal light in S band. By contrast, a Raman amplifier supplies an optical transmission line with Raman amplification pumping light having a wavelength appropriately selected according to the wavelength of signal light, thereby Raman-amplifying signal light propagating through the optical transmission line, whereas the signal light to be Raman-amplified may have any wavelength. Therefore, while the signal light in C and L bands is optically amplified by the EDFA, the signal light in S band may be optically amplified by the Raman amplifier.
For attaining a larger volume, it is desirable that both of the absolute value of chromatic dispersion and the absolute value of dispersion slope in the signal light wavelength band be smaller in an optical transmission line. However, an optical transmission line in which the respective absolute values of chromatic dispersion and dispersion slope are smaller is hard to be constructed by a single kind of optical fiber alone. Therefore, in general, two or more kinds of optical fibers (e.g., a transmission optical fiber and a dispersion-compensating optical fiber for compensating for the chromatic dispersion of this transmission optical fiber) having respective chromatic dispersion characteristics different from each other are connected to each other, so as to construct an optical transmission line in which each of the respective absolute values of chromatic dispersion and dispersion slope in the whole optical transmission line is small.
For Raman-amplifying the signal light in S band, the Raman amplification pumping light is required to have a wavelength falling within the band of 1.4 μm. However, the dispersion-compensating optical fiber used for compensating for the chromatic dispersion of the transmission optical fiber has an excess loss caused by OH group in the 1.4-μm wavelength band. Therefore, the Raman amplification pumping light in the 1.4-μm wavelength band capable of Raman-amplifying the signal light in S band incurs a large loss when propagating through the dispersion-compensating optical fiber, thus exhibiting a low efficiency when Raman-amplifying the signal light, whereby it has been difficult to construct an optical transmission line in which each of the respective absolute values of chromatic dispersion and dispersion slope in the whole optical transmission line is small, while carrying out Raman amplification.
In order to overcome the problem mentioned above, it is an object of the present invention to provide a dispersion-compensating optical fiber and an optical transmission line which are suitable for Raman-amplifying the signal light in S band while lowering the respective absolute values of chromatic dispersion and dispersion slope therein.